The present invention relates to a protective sheath for an electrical or optical conductor or a group of electrical or optical conductors, said protective sheath being hardened with respect to X-rays. It applies in all fields where it is necessary to provide X-ray protection for the transmissions of signals by electrical or optical conductors.
In particular, the invention applies to the protection of electrical and optical conductors used in the medical, nuclear (test and power stations), aeronautical and space fields.
Electrical conductors refer to all metal wires or cables especially designed for carrying electrical currents, such as coaxial telephone cables, television cables, etc. The term optical conductors covers all optical cables or fibres for carrying light signals.
It is pointed out that an optical fibre is a very fine rod of a material (synthetic, borosilicate or silica glass) which is stretched and whose filiform appearance gives it great flexibility. This optical fibre constitutes a light guide.
In general terms, an optical fibre is constituted by a dielectric medium, which is called the core and which is generally covered by a second medium, called the optical sheath, whose refractive index is below that of the core. However, it may have more complex structures and may in particular have several optical sheath layers, as described in FR-A-2 523 376.
Electrical and optical conductors are very sensitive to X-rays. Interference leads in the case of optical conductors to a darkening and in the case of electrical conductors to a disturbed electrical signal.
In order to obviate these undesirable effects, it is consequently necessary to lower the irradiation level of said conductors by protecting them against X-rays. One of the most widely used methods for reducing doses and dose rates received by electrical and optical conductors consists of placing them in X-ray-opaque channels, which are generally made from a metal with a high atomic number.
The metal and thickness of said channels are chosen and adapted as a function of the energy of the X-radiation in question and of the desired filtering level. These metal channels provide a very effective X-ray protection. Unfortunately in the case of electrical or optical conductors carried on board aircraft, said metal channels are much too heavy and cumbersome.
Moreover, it is difficult to route the electrical or optical conductors outside a clearly defined circuit, established as a function of the available locations for the channels. These positioning constraints mean that these channels are very costly. Moreover, it is difficult to work and use the most interesting metals for providing an effective X-ray protection, which further increases the cost of said channels.
More recently, in the case of electrical conductors, an X-ray protective sheath has been formed from one, two or even three braids of an alloy of a metal having a high atomic number and then covered by a silver layer. These protective sheaths are placed round the electrically insulating sheath surrounding the conductors.
Unfortunately the braiding method can leave "holes" with respect to the X-rays, which leads to a dispersion of said radiation. Moreover, it is disadvantageous from the weight standpoint, which can be highly prejudicial in connection with electrical conductors carried by aircraft.
Finally, one of the advantages of links by optical fibres is that of permitting a dielectric connection between boxes or cases. However, this advantage is lost with the braiding method.
The present invention relates to a protective sheath for electrical or optical conductors hardened with respect to X-rays and making it possible to obviate the various. disadvantages referred to hereinbefore. In particular, said sheath permits an important gain as regards weight and overall dimensions, whilst still providing an effective protection against radiation with a high dose rate. Moreover, this X-ray protection sheath causes no particular manufacturing problem.